ing, a link on the enzyme isn't held in place. This skews the reactive group out of position and no reaction occurs, even though the substrate is the same size as the proper one. Leaving out a group so the substrate is smaller than the right one also causes misalignment. The smaller chemical can be absorbed on the enzyme surface but it won't react because the reactive part isn't in the right place. The new theory, points out Koshland, incorporates features of the template theory—in both, die substrate must fit. The big difference: In the "induced fit" theory, the fit occurs only after changes in the enzyme structure. Actually, says Koshland, "We're only on the first lap of a fairly long run" in getting a complete picture of how enzymes work. But a clearer understanding of the way enzymes achieve specificity will help explain how living systems are regulated. Then, too, the door may be opened t o some real progress in chemical theory. No man-made catalyst, says Koshland, has been able to duplicate an enzyme's specificity.
Rotation and NMR N M R detects bond d i f f e r ences in isomers, allows c a l culation of barriers t o internal rotation
m
ACS NATIONAL
MEETING Physical & Inorganic Chemistry
When
chemists
learn enough about x.1.
i -
the inner workings of known molecules, they can feel more sure-footed about extending what they know to more complex molecules and, indeed, to predicting properties of molecules as yet unknown. These ideas are a major driving force behind all spectroscopy and, specifically, behind a nuclear magnetic resonance program that D u Pont has been pressing for the past four years, according to W . D . Phillips. One subject the D u Pont people are probing is hindered rotation in molecules. The two general aims: • To learn what the facts shown by N M R mean in terms of hindered rotation. • To learn to relate these NMR data to energy barriers to internal rotation, and to things like molecular geometry and electronic structure. NMR has proved extremely useful in
this • work because it can distinguish between the different types of bonds present in different stereoisomers of a particular molecule. But if the isomers switch from one form to the other faster than roughly 1000 times per second, NMR won't show discrete peaks, Phillips told the Symposium on Nuclear Magnetic Resonance Spectroscopy, sponsored jointly by the Organic and the Physical and Inorganic Divisions. Sometimes this problem can be evaded by cooling the system, thus slowing the rate of reorientation. It turns out, says Phillips, that this temperature transition point (in terms of whether NMR works or not) can be pinned down quite accurately. And by studying NMR spectra around that point, one can figure out the energy barriers to rotation about bonds. Sometimes, as with nitrosamines, the system must be heated to get it close enough to the transition point to do this. • It's Partly Double. In nitrosamines rotation is hindered around the N—N bond, which has long been written as a single bond. Hindrance around a single bond should be low, but NMR shows that it's quite high. This means, says Phillips, that the bond behaves, in part, like a double bond—i.e., dislocated electrons result in pi bonding, as in benzene. NMR also shows such a bond (O—N) in alkyl nitrites. Here, NMR can show the temperature dependence of the cis/trans isomer ratio, and thus the energy difference between the two forms. It comes out to be rather low, confirming that the barrier to rotation around the O—N bond is due to its partial double bond nature, rather than to hydrogen bonding, as had been assumed. In oximes, NMR shows hindered rotation about the C—N bond as expected; it's written classically as a double bond. But in oximes of straight chain aldehydes the syn/anti (analogous to cis/trans) isomer ratio is close to one, and the anti form actually predominates in acetaldoxime. This was unexpected, says Phillips, because the alkyl substituent is much bulkier than the aldehyde proton. However, certain branched aliphatic oximes seem to conform more closely to the expected pattern. NMR will detect thermal interconversion of isomeric oximes; D u Pont is working on quantitative values for the energy barrier to rotation about the C - N bond. Ethanes have, in general, three isomeric forms. But barriers (van der Waals forces) to rotation around the
C—C bond are quite low, so the isomers switch back and forth too fast for NMR. In a few cases, says Phillips, they've managed to whip this problem Ly dropping the temperature. Below —80° C , for instance, NMR shows two rotational isomers of BrF 2 C—CBr 2 CN. The asymmetrical form seems about twice as populous as the symmetrical form; the energy difference between the two is relatively small, the asymmetrical form the more stable.
Solid Monolayer Films Lubricate Adsorbed monornolecular polar compounds a r e keys to good lubricity a n d w e a r prevention
m
A thin, protec-
ACS
NATIONAL _ _._ _AAPFTIMG Colloid.
tive film of a chemi c a i addition agent is the key to good Chemistry lubrication. William A. Zisman and coworkers at the U. S. Naval Research Laboratory in Washington affirm this and say that such a coating should consist of polar molecules adsorbed as solid monolayers on rubbing surfaces. New surface chemical methods developed at NRL permit adsorption of a well-defined, closely packed, monomolecular film of any adsorbable compound and isolate this coating on the surface of metals or other smooth solids. Measurement and control of molecular packing in condensed monolayers are obtained by measuring contact angle, or wettability, of a reference liquid— methylene iodide—on a film-covered surface. NRL finds now that structure and constitution of these films is indicated by variation in coefficient of friction when lubricated surfaces continue to rub together. • Sliding-Bal! Tester Used. By sliding a polished steel ball subjected to known pressure over the same path repeatedly on a flat, lubricated surface of metal or glass, the coefficient of friction can be recorded continuously, Zisman's group told the Division of Colloid Chemistry. Also, this method is a powerful research tool for studying durability and wear-prevention of adsorbed films, they feel. On the basis of significant differences in frictional and durability behavior of adsorbed monolayers, NRL groups the many types of films studied into three SEPT.
2 3,
1957
C&EN
61
RESEARCH
classes of two-dimensional structure: liquid, plastic, solid. "Solid" monolayers are found to provide lowest coefficients of friction as well as greatest durability and wear-prevention. Mixed monolayer films containing both polar additives and hydrocarbon oil occur only when structures of the oil molecule and the hydrocarbon portion of t h e polar compound are geometrically similar. This permits strong adlineation, or intermolecular cohesion, according to NRL. While the load-carrying capacity of mixed films is much less than that of pure films, mixed monolayers occur frequently in lubrication practice, says Zisman. This is especially true in dynamic machinery where there is not enough tame between formation and destruction of a protective film to allow adsorption equilibrium to be reached. Zisman feels that this explains why load-carrying and wear-preventive properties of chemical additives to oils measure less in dynamic testing devices than in static ones. • Monolayer Films Are Strong. Friction at the boundary of two rubbing solids is due to adhesion between high spots on both surfaces. An adsorbed film serves basically a mechanical function by greatly decreasing the number of contacting rough places. A "solid" film only one ten-millionth of an inch thick on glass can withstand repeated traverses on a rubbed path by a steel ball under loads as high as 100,000 pounds per square inch, the NRL group reports. It continues to separate the rubbing surfaces, reduces coefficient of friction to 0.05, and reduces wear by a factor of 10,000. Some of the highest rough spots can always penetrate adsorbed films on rubbing surfaces, Zisman explains. For steel on glass, junctions formed between these two materials shear at points of contact, and damage is negligible until many traverses of a single rubbed path are made. Steel rubbing against steel results in strong adhesion a n d cold welding. These junctions do not shear just a t contact points but often tear relatively deep in the metal surface. Larger particles of metal which are thus torn off adhere to a sliding surface and plow through the protective monolayer. In general, then, t h e protective value of such monolayers is less for rubbing surfaces which alloy or form strong welds, NRL maintains. 62
C&EN
SEPT.
23,
1957
Chemical requirements for a good, polar wear-preventive adsorbed as a two-dimensional solid monolayer have been drawn up by Zisman and his coworkers to include t h e following four points: • Intermolecular cohesion must h e high; molecules must adsorb in oriented array, a n d paraffinic structures with one or more chemically bonded halogen atoms are desirable. • Polar terminal groups of t h e molecules must adhere strongly to t h e rubbing surface. • Easy slippage a n d low affinity b e tween films on contacting surfaces must be assured; outermost terminal groups of oriented molecules should form a surface having lowest possible free surface-energy (methyl or perfluoromethyl groups are ideal) . • Rate of diffusion to adsorbing surface must not be too slow, a n d molecules should not be too bulkv.
Hydrofhermal Sapphires Sapphires g r o w n in aqueous solution seem f r e e of strain, might thus b e used in optics
•moACs I » # NATIONAL I \ 3 L MEETING
Taking
a
cue
from its h y d r o t h e r ] process for PhysWi* synthetic quartz, Inorganic Bell Telephone Chemistry L a b s has grown synthetic sapphires ( c o r u n d u m — a modification o f A l 2 O a ) a s large as s / 4 X V 4 X V 4 i n c h . U n like flame fusion ( t h e p r e s e n t commercial p r o c e s s ) a n d m e l t t e c h n i q u e s , t h e n e w p r o c e s s gives crystals t h a t s e e m essentially f r e e of s t r a i n , say R . A. L a u dise a n d A. A . Ballman. S u c h crystals m a y p r o v e useful i n lenses, prisms, a n d o t h e r optical items. Some n a t u r a l s a p p h i r e is used for such t h i n g s now—but a t a price—because it transmits l i g h t well into the infrared a n d , e s p e c i a l l y , t h e v a c u u m ultraviolet. A b r a s i v e s u s e m o s t of the c o m m e r c i a l c o r u n d u m m a d e now. H y d r o t h e r m a l p r o c e s s e s like this one exploit t h e f a c t t h a t normally insoluble c h e m i c a l s c a n b e c o m e slightly soluble if p r e s s u r e a n d t e m p e r a t u r e go h i g h e n o u g h . F o r instance, the Bell p r o c ess o p e r a t e s a t a b o v e 4 0 0 ° C . and 30> 0 0 0 p.s.i., L a u d i s e told the D i v i s i o n of ma
P h y s i c a l asud fbaorgjanniic Oasnniiistnrvr. Briefly, B r e l xo-ak-es sapplhine Ibv taili n g a >maIL silver-limed! aniriicxdLa^ie, cos* a silver t u b e ± o s i d e nit tfe© TiQ) &© *&&** m i t s free vohicmae ^arMa conae tmnrfpilgur sflHflnaiunia carbonate.. BseasgeauSt aloflnaaiinita «KT alham i n n m h y d r o s i & e •§© an tribe Ibwsaitaffinm ©IF t h e r e a c t i o n c a v i t y ; s e e d cirystaBs, if (ctomomiMSminn r a t h e r fhatn de^ositiiniEg IQHH iiikke seseafeT o explaini all ttiaas ithe Ee33 jpxssgBBe p o s t u l a t e t h a t s e e d ttfioey ha\ T en"t t r i e d i t , stair s a p p h i r e s nnaas&itt fee m a d e h v -addicti^ tittaiiasia anttd ninonn-
